There's no reasonable way to give a single answer - which is why Epson's figures are considered conservative and why you have a lot of stories about them lasting well beyond the listed dates and basically no stories of them going bad early.

It depends on a number of environmental considerations (heat, light, pressure, humidity, transport) so they will last longer in some environments compared to others. Essentially, the Epson guidelines are "worst case scenario".

If Epson replaces the 3880 this year it will likely be announced in late summer, available at full price around December, and rebated price will hit in March or April. Epson has no major competition in that size printer right now so they may keep the 3880 current for another year IMO.